The present invention relates to a process for the preparation of 2-acrylamido-2-methyl-1-propanesulfonic acid. More particularly the said process is related to the preparation of highly pure 2-acrylamido-2-methyl-1-propanesulfonic acid in high yield, with improved appearance, by the reaction of acrylonitrile with more than 98% sulfuric acid or oleum and liquefied isobutylene in presence of weak inorganic acids or organic sulfonic acids.
2-acrylamido-2methyl-1-propane-sulfonic acids as well as its homologues are used for improving the affinity of the acrylic fibers, towards dyes, in the dying process. 2-acrylamido-2-methyl-1-propane-sulfonic acid and its homologues function as co-monomers during the polymerization process with acrylic monomers. Further, the homo-polymers or the copolymers of 2-acrylamido-2-methyl-1-propane sulfonic acid as well as its homologues, are used as extremely superior high molecular electrolytes and dispersants, thickeners, power failure prevention agents (2nd Gazette of American Patents No. 298371, Gazette 3332904 of the same patent). Apart from this, 2-acrylamido-2-methyl-1-propane-sulfonic acid as well as its congeners are also used as polymers with water absorption properties.
The manufacturing method for 2-acrylamido-2-methyl-1-propane-sulfonic acid as is known from the prior art, in general, comprises two steps. In step 1, acrylonitrile is reacted with concentrated sulfuric acid or oleum in presence of an additive at low temperatures. The resultant mixture is treated with isobutylene gas in the second step. Various methods of manufacturing 2-acrylamido-2-methyl-1-propane-sulfonic acid based on above general method have been suggested in the prior art.
Japanese Patent No JP 4074159 discloses the manufacture of 2-acrylamido-2-methyl-1-propanesulfonic acid by treating isobutylene gas with acrylonitrile and more than 95% conc. H2SO4 in the presence of organic carboxylic acids or their anhydrides. A mixture of 375 g acrylonitrile, and AcOH was treated drop wise with 98 g 100% H2SO4 at less than 0xc2x0 C., introduced with 56 g of isobutylene gas at 30xc2x0 C. over 52 min, then settled at room temperature for 1 h to give 186 g of 2-acrylamido-2-methyl-1-propane-sulfonic acid of 98.9% purity with APHA 20.
This method, while suitable for industrial production, referring to the examples cited in the above mentioned prior art, the yield is higher at 91.3% of theoretical on isobutylene, with corresponding purity of final product 2-acrylamido-2-methyl-1-propansulfonic acid is only 99.3%. Similarly, although the molar ratio of acrylonitrile to sulfuric acid is claimed to be greater than or equal to 4, all the examples use the acrylonitrile in the proportion of excess of 8.3 moles per mole of sulfuric acid. Such use of excess of acrylonitrile in the process leads to higher capital investment on recovery as well as higher operating cost of recovery at the industrial scale of manufacture. Furthermore, this method requires the reaction to be carried out at below 0xc2x0 C. Therefore the cooling medium has to be provided at below sub-zero temperature resulting in increase in the cost of the utilities. Similarly, the total time required in this method for step 1 is 52 minutes while that for the step 2 is 1 hour. Thus, the reaction has to be carried out for a long time exceeding the total time by more than 112 minutes for both steps.
The use of organic carboxylic acids during the synthesis of 2-acrylamido-2-methyl-1-propane-sulfonic acid also leads to the formation of a thick slurry of the reaction mixture which poses difficulties in stirring as well as filtration. The quality of 2-acrylamido-2-methyl-1-propanesulfonic acid obtained in terms of purity and color specifications is also not satisfactory for the applications as mentioned above.
Japanese Patent No. JP 03077859 A2, discloses a method for manufacture of 2-acrylamido-2-methyl-1-propane-sulfonic acid from acrylonitrile, concentrated or fuming H2SO4 and isobutylene in the presence of various stabilizers
A) Formula I
R1xe2x80x94NHxe2x80x94R2xe2x80x83xe2x80x83Formula I
where R1, R2 are the same or different and are selected from hydrogen, amino, alkyl and alkylaryl or phenyl, lower alkyl having amino group as a substitute group or salts thereof. or B) Formula II 
where R1 and R2 are same as above; R1 and R2 can be xe2x80x94CH2, CH2xe2x80x94NHxe2x80x94, R3 is selected from hydrogen, lower alkyl group, OH or xe2x80x94N=C (Me); Yxe2x95x90O, S; or salts thereof or C) and adding inorganic reducing agents and, if necessary, solubilizers.
Although this method is also good for industrial production, the drawbacks are that, the isobutylene has to be used in gaseous form. In a continuous process, for handing gaseous isobutylene, the reactor has to be made of a specialized configuration to minimize the escape of unreacted isobutylene gas. This calls for a special arrangement which increases the capital cost of the hardware. Furthermore, the introduction of compounds represented by Formula I an Formula II as stabilizers, and the inorganic salts as solubilizers cause increase in the raw material cost and the processing costs leading also to contamination of the final product.
German Patent No. Ger. Offen. DE 290465 discloses a method for the manufacture of highly pure 2-acrylamido-2-methyl-1-propanesulfonic acid. High purity 2-acrylamido-2-methyl propanesulfonic acid (I), useful in the manufacture of dyeable acrylic polymers and of polyelctrolytes, is obtained by adding AcOH to crystalline Ixe2x80x94containing slurry prepared from acrylonitrile, isobutylene, and concentrated or fuming H2SO4 or anhydrous sulfuric acid in a reaction medium, distilling to replace the reaction medium with AcOH, adding water or aq. H2SO4, heating to dissolve the sulfonic acid of formula I, and crystallizing it from the solution.
Although this method is simpler it involves additional steps of replacing the reaction medium with AcOH, heating and dissolution and recrystallization of product from the solution. These steps increase the cost of the process and the investment on industrial scale. In addition the yield loss due to the additional refining stages makes the process more uneconomical.
Japanese Patent No. JP 56103145 A2 discloses a method for manufacture of vinlyamides. Vinylamides of the formula H2C: CRCONHCMeR1R2 I (R, R1, R2=H, Me, CH2SO3Na; H, H, C6H4SO3Naxe2x88x924; H, Me CH2OC6H4SO3Naxe2x88x924;H, H, CH2OC6H4SO3Naxe2x88x924; Me, Me, CH2SO3Na; H, Me, CH2SO3H; H, Me, CH2CI) were prepared. Thus, 202 g greater than 97% H2SO4 in AcOh was added to a mixture of H3C:CmeCH2SO3Na 158, H20 27, H2C:CHCN 106 g at  greater than 20 over 10 h to give, after 38 h, 195 g I (R=H, R1=Me, R2).
It is evident from this information that this method requires very long time of reaction, in excess of 48 hours. Therefore this method does not give any economical advantage on industrial scale.
Japanese Patent No. JP 02096559 A2 9, discloses a method for preparation of amidoalkanesulfonic acids as dispersants for calcium soaps as R1CONHCR2R3CR4R5SO3H [R1=hydrocarbyl; R2 to R5=H, (substituted) hydrocarbyl], useful as dispersants for Ca soaps, etc. (no data), or their precursors are prepared by treating nitriles and olefins with SO3, H2SO4, or oleum and optionally with H2O in presence of 1 wt. % (per nitriles) carboxamides. Thus, a 573.0:12.6 (wt. ratio) mixture of CH2:CHCN and CH2:CHCONH2 was treated dropwise with oleum at 0-5xc2x0 C. and stirred with Me2C:CH2 (I) at 50xc2x0 C. for 1 h, and H2O was added to the reaction mixture to give 79.7% (based on I), CH2:CHCONHCMe2CH2SO3H of 98.2% purity and APHA 5 (25% aq. Solution).
This method has the evident drawbacks of low purity and as well as low yields. Hence the product obtained by this method is not likely to be sufficient in quality for the applications mentioned above. And the process may not be economical on industrial scale due to poor yields.
The main object of the present invention is to provide a process for preparation of 2-acrylamido-2-methyl-1-propanesulfonic acid.
Another object of the present invention is to reduce the proportion of acrylonitrile compared to that of sulfuric acid in the reaction.
Still another object of the present invention is to avoid any additional raw material in the form of a stabilizer or solubilizer in the process.
Yet another object of the present invention is to carry out the process at temperatures greater the 0xc2x0 C.
A further another object of the present invention is to reduce the total residence time of the reaction as low as possible.
Another object of the present invention is to provide a process for manufacture of 2-acrylamido-2-methyl-1-propane-sulfonic acid which obviates the difficulties in stirring and filtration during processing of 2-acrylamido-2methyl-1-propane-sulfonic acid.
Accordingly the present invention provides a process for preparation of 2-acrylamido-2-methyl-1-propane-sulfonic acid, which comprises mixing acrylonitrile with a mixture of sulfuric acid and a catalyst maintaining a maximum temperature of 20xc2x0 C., followed by addition of isobutylene maintaining a maximum temperature of 50xc2x0 C. and separating the product formed, purifying the product by washing with acrylonitrile followed by drying under vacuum at temperature 50 to 60xc2x0 C.
In one embodiment of the invention strength of sulfuric acid is not less than 98%.
In another embodiment the catalyst is a weak inorganic acid such as othophosphoric acid, or an organic sulfonic acid selected from paratoluene sulfonic acid and benzene sulfonic acid, preferably partatoluene sulfonic acid.
In still another embodiment the ratio of acrylonitrile to mixture of sulfuric acid and catalyst is in the range of 3:1 to 6:1.
In yet another embodiment the ratio of catalyst to acrylonitrile is in the range of 0.02 to 100 to 4.0 to 100, preferably 0.3 to 1.
In still another embodiment the residual time for the retention of acrylonitrile with mixture of sulfuric acid and catalyst is in the range of 10 to 15 minutes, preferably 15 minutes.
In yet another embodiment isobutylene is added within a period of 10 to 15 minutes preferable 15 minutes.